The embodiments described herein relate generally to a photovoltaic (PV) power generation system, and more specifically, to systems for coupling multiple variable input single-phase direct current (DC) power sources to a symmetric three-phase alternating current (AC) grid.
Solar energy has increasingly become an attractive source of energy and has been recognized as a clean, renewable alternative form of energy. Solar energy in the form of sunlight may be converted to electrical energy by solar cells. A more general term for devices that convert light to electrical energy is “photovoltaic cells.” Sunlight is a subset of light. Thus, solar cells are a subset of photovoltaic (PV) cells. A PV cell comprises a pair of electrodes and a light-absorbing PV material disposed therebetween. When the PV material is irradiated with light, electrons that have been confined to an atom in the PV material are released by light energy to move freely. Thus, free electrons and holes are generated. The free electrons and holes are efficiently separated so that electric energy is continuously extracted. Current commercial PV cells use a semiconductor PV material, typically silicon.
In order to obtain a higher current and voltage, solar cells are electrically connected to form a solar module. In addition to a plurality of solar cells, the solar module may also include sensors, for example, an irradiance sensor, a temperature sensor, and/or a power meter. Solar modules may also be connected to form a module string. Typically, the DC voltages output by the module strings are provided to a grid inverter, for example, a DC to AC voltage inverter. The DC to AC voltage inverter converts the DC voltage to a single or three-phase alternating current (AC) voltage or current. The three-phase AC output can be provided to a power transformer, which steps up the voltage to produce a three-phase high-voltage AC that is applied to an electrical distribution grid.
Electricity applied to the electrical distribution grid is required to meet grid connectivity expectations. These requirements address safety issues as well as power quality concerns. For example, the grid connectivity expectations include facilitating disconnecting the power generation system from the grid in the event of a transient event, for example, a power surge or power failure. Another grid connectivity expectation is that the generated power be conditioned to ensure that the power matches the voltage and frequency of the electricity flowing through the grid. For example, the Institute of Electrical and Electronics Engineers (IEEE) has written a standard that addresses grid-connected distributed generation including renewable energy systems (IEEE 1547-2003). Underwriters Laboratories (UL) has also developed a standard, UL 1741, to certify inverters, converters, charge controllers, and output controllers for power-producing stand-alone and grid-connected renewable energy systems. UL 1741 verifies that inverters comply with IEEE 1547 for grid-connected applications.
Specifically, a grid-connected PV power generation system must meet utility interconnection requirements including low voltage ride through (LVRT), voltage regulation, and power factor correction.